「Skyrmions, Merons and Monopoles: Topological Excitations in Chiral Magnets」
Avadh Saxena　氏

日本物理学会北海道支部講演会

講演題目：　Skyrmions, Merons and Monopoles: Topological Excitations in Chiral Magnets
講　師　：　Avadh Saxena　博士
　　　　　Los Alamos National Lab
日　時　：　平成28年4月28日 (木) 16:30-18:00
場　所　：　北海道大学 工学部 物理工学系大会議室 (A1-17室)

要　旨　：
Stable topological excitations such as domain walls and vortices are ubiquitous in condensed matter as well as high energy physics and are responsible for many emergent phenomena. In 2009 a new mesoscopic spin texture called skyrmion was discovered experimentally in certain conducting and insulating magnets. It is now believed to exist in Bose-Einstein condensates, 2D electron gases, superconductors, nematic liquid crystals among many other systems. This topological excitation was originally proposed by Tony Skyrme in 1958 in a nonlinear field theory of baryons. In the temperature--‐magnetic field phase diagram of chiral magnets, skyrmions form a triangular lattice in the low temperature and intermediate magnetic field region (in thin films). In metallic magnets, skyrmions can be driven by a spin polarized current while in insulating magnets by magnons. The threshold current density to depin skyrmions is 4 to 5 orders of magnitudes weaker than that for magnetic domain walls. The low depinning current makes skyrmions extremely promising for applications in spintronics. I will first attempt to summarize the experiments and present an overview on skyrmions. Then I will demonstrate how increasing the easy-plane anisotropy results in a transition from a triangular lattice of skyrmions to a square lattice and eventually to merons. The latter are essentially half- skyrmions and with half the topological charge. Finally, I will show that under current driving skyrmions tubes can split or merge at certain points leading to the formation of magnetic monopoles and anti--‐monopoles connected by a Dirac string. “Observation of a quantum Cheshire Cat in a matter wave interferometerexperiment” T. Denkmayr, H. Geppert, S. Sponar, H. Lemmel, A. Matzkin,J. Tollaksen, and Y. Hasegawa, quant-ph/1312.3775, Nature Comm. 5:4492doi: 10.1038 / ncomms5492 (2014).

世話人　 丹田 聡

北海道大学工学部応用理工系学科　(電話011-706-6115)

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